On the residence time for water in a solute hydration shell: Application to aqueous halide solutions

We investigate several different methods to determine the water residence time next to a solute from molecular dynamics simulations. The popular computational prescription due to Impey et al. (J. Phys. Chem. 1983, 87, 5071-5083) is shown to be. extremely sensitive to the t* tolerance time value (designed to account for barrier recrossing effects), and we evidence through a kinetic analysis that the conventionally employed t* = 2 ps value can yield seriously overestimated residence times for low barrier exchanges. We suggest an alternate, robust determination based on the stable states picture (SSP) of chemical reactions (J. Chem. Phys. 1980, 73, 2700-2714) where recrossing is naturally discarded. This is illustrated by calculation of the water residence time next to a water molecule and next to a chloride ion, using both nonpolarizable and polarizable force-fields. The SSP results are in good agreement with the residence times estimated by a separate kinetic analysis, and differ noticeably from those calculated in the conventional fashion mentioned above.